I was thrown by the p2 when trying to figure it out too, because I was assuming that it was part of the debian revision. In fact, it is part of the upstream version, which you can confirm by following the link to sudo development below.

Finally, you can confirm this order of package releases on the Debian tracker, also linked below. The developer accepted sudo 1.821-1 to unstable on 2017-08-28, then version 1.8.21p2-1 on 2017-09-07. Your current candidate should definitely contain the fix for the postinst script.